U.S. patent number 11,156,848 [Application Number 16/680,704] was granted by the patent office on 2021-10-26 for wavelength beam combining laser systems with high beam quality factor.
This patent grant is currently assigned to TERADIODE, INC.. The grantee listed for this patent is Bien Chann, Michael Cruz, Robin Huang, Parviz Tayebati, Wang-Long Zhou. Invention is credited to Bien Chann, Michael Cruz, Robin Huang, Parviz Tayebati, Wang-Long Zhou.
United States Patent |
11,156,848 |
Tayebati , et al. |
October 26, 2021 |
Wavelength beam combining laser systems with high beam quality
factor
Abstract
In various embodiments, optical repositioners and/or angled
dispersive elements are utilized to manipulate portions of an input
laser beam emitted by a group of laser emitters in order to form a
multi-wavelength output beam having a high beam quality factor.
Inventors: |
Tayebati; Parviz (Sherborn,
MA), Zhou; Wang-Long (Andover, MA), Chann; Bien
(Merrimack, NH), Huang; Robin (North Billerica, MA),
Cruz; Michael (Wilmington, MA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Tayebati; Parviz
Zhou; Wang-Long
Chann; Bien
Huang; Robin
Cruz; Michael |
Sherborn
Andover
Merrimack
North Billerica
Wilmington |
MA
MA
NH
MA
MA |
US
US
US
US
US |
|
|
Assignee: |
TERADIODE, INC. (Wilmington,
MA)
|
Family
ID: |
54055899 |
Appl.
No.: |
16/680,704 |
Filed: |
November 12, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20200174264 A1 |
Jun 4, 2020 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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14640079 |
Mar 6, 2015 |
10514549 |
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61949226 |
Mar 6, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
5/08 (20130101); G02B 27/14 (20130101); F21V
13/04 (20130101); H01S 5/143 (20130101); H01S
5/4062 (20130101); G02B 27/106 (20130101); H01S
3/10 (20130101); G02B 7/003 (20130101); G02B
27/1006 (20130101); H01S 5/02251 (20210101); H01S
5/4056 (20130101); H01S 5/4031 (20130101); H01S
5/405 (20130101); H01S 3/094076 (20130101) |
Current International
Class: |
G02B
27/10 (20060101); H01S 5/40 (20060101); F21V
13/04 (20060101); G02B 5/08 (20060101); G02B
27/14 (20060101); H01S 3/10 (20060101); G02B
7/00 (20210101); H01S 5/14 (20060101); H01S
3/094 (20060101); H01S 5/02251 (20210101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
PCT International Patent Application No. PCT/US2015/019116,
International Search Report and Written Opinion dated Jun. 30,
2015, 8 pages. cited by applicant.
|
Primary Examiner: Thomas; Brandi N
Attorney, Agent or Firm: Morgan, Lewis & Bockius LLP
Parent Case Text
RELATED APPLICATION
This application is a division of U.S. patent application Ser. No.
14/640,079, filed Mar. 6, 2015, which claims the benefit of and
priority to U.S. Provisional Patent Application No. 61/949,226,
filed Mar. 6, 2014, the entire disclosure of each of which is
hereby incorporated herein by reference.
Claims
What is claimed is:
1. A laser apparatus comprising: a plurality of laser emitters
arranged in an array and each emitting a laser beam at a different
wavelength, the plurality of laser beams collectively forming an
array having first and second orthogonal dimensions; focusing
optics for receiving the plurality of laser beams and combining the
plurality of laser beams along the first dimension to form a
combined beam; a plurality of dispersive elements positioned to
receive the combined beam and transmit a multi-wavelength beam,
each dispersive element (i) being disposed at a different angle
with respect to the combined beam, and (ii) transmitting a band of
the multi-wavelength beam being composed of a different set of
wavelengths; a partially reflective output coupler positioned to
receive the multi-wavelength beam, transmit a portion of the
multi-wavelength beam therethrough, and reflect a second portion of
the multi-wavelength beam back toward the plurality of dispersive
elements; and a plurality of dichroic elements positioned to (i)
receive the portion of the multi-wavelength beam transmitted by the
partially reflective output coupler, (ii) separate the portion of
the multi-wavelength beam into the plurality of bands, and (iii)
spatially overlap the separated bands into a final overlapped
beam.
2. The laser apparatus of claim 1, wherein each dispersive element
comprises a diffraction grating.
3. The laser apparatus of claim 1, wherein the focusing optics
comprises at least one of a cylindrical lens or a cylindrical
mirror.
4. The laser apparatus of claim 1, wherein each dichroic element
comprises a dichroic mirror, a dichroic filter, or an interference
filter.
5. The laser apparatus of claim 1, wherein the plurality of
dispersive elements are aligned along a beam-combining line of the
combined beam.
6. The laser apparatus of claim 1, wherein the array is a
one-dimensional array.
7. The laser apparatus of claim 1, wherein the array is a
two-dimensional array.
8. The laser apparatus of claim 1, wherein each dispersive element
comprises a dispersive prism, a grism, or an Eschelle grating.
9. The laser apparatus of claim 1, further comprising a workpiece
to which the final overlapped beam is transmitted.
10. The laser apparatus of claim 1, further comprising an optical
fiber to which the final overlapped beam is transmitted.
11. The laser apparatus of claim 1, wherein the output coupler is
positioned to reflect the second portion of the multi-wavelength
beam back toward the plurality of dispersive elements and thence to
the plurality of laser emitters, whereby an external lasing cavity
is formed.
12. The laser apparatus of claim 1, wherein a size of the final
overlapped beam is equal to a size of one of the bands.
13. The laser apparatus of claim 1, wherein the plurality of
dispersive elements comprises at least three dispersive elements.
Description
TECHNICAL FIELD
In various embodiments, the present invention relates to laser
systems, specifically wavelength beam combining laser systems with
improved beam quality factor.
BACKGROUND
High-power laser systems are utilized for a host of different
applications, such as welding, cutting, drilling, and materials
processing. Such laser systems typically include a laser emitter,
the laser light from which is coupled into an optical fiber (or
simply a "fiber"), and an optical system that focuses the laser
light from the fiber onto the workpiece to be processed. The
optical system is typically engineered to produce the
highest-quality laser beam, or, equivalently, the beam with the
lowest beam parameter product (BPP). The BPP is the product of the
laser beam's divergence angle (half-angle) and the radius of the
beam at its narrowest point (i.e., the beam waist, the minimum spot
size). The BPP quantifies the quality of the laser beam and how
well it can be focused to a small spot, and is typically expressed
in units of millimeter-milliradians (mm-mrad). A Gaussian beam has
the lowest possible BPP, given by the wavelength of the laser light
divided by pi. The ratio of the BPP of an actual beam to that of an
ideal Gaussian beam at the same wavelength is denoted M.sup.2, or
the "beam quality factor," which is a wavelength-independent
measure of beam quality, with the "best" quality corresponding to
the "lowest" beam quality factor of 1.
Wavelength beam combining (WBC) is a technique for scaling the
output power and brightness from laser diode bars, stacks of diode
bars, or other lasers arranged in one- or two-dimensional array.
WBC methods have been developed to combine beams along one or both
dimensions of an array of emitters. Typical WBC systems include a
plurality of emitters, such as one or more diode bars, that are
combined using a dispersive element to form a multi-wavelength
beam. Each emitter in the WBC system individually resonates, and is
stabilized through wavelength-specific feedback from a common
partially reflecting output coupler that is filtered by the
dispersive element along a beam-combining dimension. Exemplary WBC
systems are detailed in U.S. Pat. No. 6,192,062, filed on Feb. 4,
2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat. No.
8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107,
filed on Mar. 7, 2011, the entire disclosure of each of which is
incorporated by reference herein.
While a variety of WBC techniques have been utilized to form
high-power lasers for a host of different applications, many such
techniques involve complicated arrangements of optical elements for
beam manipulation, and, depending on the locations of the various
optical elements in the optical train of the system, it may be
difficult to obtain the desired beam quality factor of the final
combined beam. Thus, there is a need for simplified WBC systems and
techniques of combining the outputs of different laser emitters
into an output beam having a high beam quality factor (i.e., a beam
quality factor as close to unity as possible).
SUMMARY
In accordance with embodiments of the present invention, WBC
systems and techniques manipulate multiple input beams in different
portions (which in some cases each correspond to a unique set of
wavelengths) that are recombined into a multi-wavelength output
beam with a high beam quality factor. In some embodiments, optical
repositioners are utilized to redirect and/or rotate portions of a
combined input beam upstream of a dispersive element.
Advantageously, the optical repositioners are disposed downstream
of the focusing optics that combine the beams in one dimension,
obviating the need for more complicated arrangements and additional
focusing optics (e.g., ensure collimation of multiple beams prior
to the focusing optics).
In other embodiments, the dispersive element is replaced by a group
of dispersive elements each angled differently with respect to the
input beam. Each dispersive element thus transmits a different
multi-wavelength "band" of different wavelengths in the output
beam. These bands may advantageously be separated, recombined,
and/or overlapped via the use of dichroic elements in order to form
an output beam having a high beam quality factor.
Embodiments of the present invention couple the one or more input
laser beams into an optical fiber. In various embodiments, the
optical fiber has multiple cladding layers surrounding a single
core, multiple discrete core regions (or "cores") within a single
cladding layer, or multiple cores surrounded by multiple cladding
layers.
Herein, "optical elements" may refer to any of lenses, mirrors,
prisms, gratings, and the like, which redirect, reflect, bend, or
in any other manner optically manipulate electromagnetic radiation.
Herein, beam emitters, emitters, or laser emitters, or lasers
include any electromagnetic beam-generating device such as
semiconductor elements, which generate an electromagnetic beam, but
may or may not be self-resonating. These also include fiber lasers,
disk lasers, non-solid state lasers, etc. Generally, each emitter
includes a back reflective surface, at least one optical gain
medium, and a front reflective surface. The optical gain medium
increases the gain of electromagnetic radiation that is not limited
to any particular portion of the electromagnetic spectrum, but that
may be visible, infrared, and/or ultraviolet light. An emitter may
include or consist essentially of multiple beam emitters such as a
diode bar configured to emit multiple beams. The input beams
received in the embodiments herein may be single-wavelength or
multi-wavelength beams combined using various techniques known in
the art.
In an aspect, embodiments of the invention feature a laser
apparatus that includes or consists essentially of a plurality of
laser emitters, focusing optics, a plurality of optical
repositioners, a dispersive element, and a partially reflective
output coupler. The laser emitters are arranged in an array and
each emit a laser beam at a different wavelength. The plurality of
laser beams collectively form an array having first and second
orthogonal dimensions. The focusing optics receive the plurality of
laser beams, combine the plurality of laser beams along the first
dimension to form a combined beam, and focus the combined beam
toward the dispersive element. Each optical repositioner is
positioned to receive only a portion of the combined beam along the
second dimension. An unintercepted portion of the combined beam
propagates to the dispersive element. Each optical repositioner
redirects a received portion of the combined beam to overlap with
the unintercepted portion of the combined beam along the first
dimension (e.g., at the dispersive element). The distance between
the dispersive element and the focusing optics may approximately
correspond to a focal length of the focusing optics. The plurality
of optical repositioners is disposed between the focusing optics
and the dispersive element. The dispersive element receives, and
transmits as a multi-wavelength beam, the received portions of the
combined beams and the unintercepted portion of the combined beam.
The partially reflective output coupler is positioned to receive
the multi-wavelength beam, transmit a portion of the
multi-wavelength beam therethrough, and reflect a second portion of
the multi-wavelength beam back toward the dispersive element.
Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The dispersive
element may include or consist essentially of a diffraction
grating. Each optical repositioner may include or consist
essentially of one or more reflectors and one or more optical
alignment elements. The focusing optics may include or consist
essentially of a cylindrical lens and/or a cylindrical mirror. Each
received portion of the combined beam may include or consist
essentially of one-third of the combined beam.
In another aspect, embodiments of the invention feature a laser
apparatus that includes or consists essentially of a plurality of
laser emitters, focusing optics, a plurality of dispersive
elements, a partially reflective output coupler, and a plurality of
dichroic elements. The plurality of laser emitters is arranged in
an array. Each laser emitter emits a laser beam at a different
wavelength, the plurality of laser beams collectively forming an
array having first and second orthogonal dimensions. The focusing
optics receive the plurality of laser beams and combine the
plurality of laser beams along the first dimension to form a
combined beam. The plurality of dispersive elements is positioned
to receive the combined beam and transmit a multi-wavelength beam.
Each dispersive element is disposed at a different angle with
respect to the combined beam and transmits a band of the
multi-wavelength beam being composed of a different set of
wavelengths. The partially reflective output coupler is positioned
to receive the multi-wavelength beam, transmit a portion of the
multi-wavelength beam therethrough (i.e., through the partially
reflective output coupler), and reflect a second portion of the
multi-wavelength beam back toward the plurality of dispersive
elements. The dichroic elements are positioned to (i) receive the
portion of the multi-wavelength beam transmitted by the partially
reflective output coupler, (ii) separate the portion of the
multi-wavelength beam into the plurality of bands, and (iii)
spatially overlap the separated bands into a final overlapped
beam.
Embodiments of the invention may include one or more of the
following in any of a variety of combinations. Each dispersive
element may include or consist essentially of a diffraction
grating. The focusing optics may include or consist essentially of
a cylindrical lens and/or a cylindrical mirror.
These and other objects, along with advantages and features of the
present invention herein disclosed, will become more apparent
through reference to the following description, the accompanying
drawings, and the claims. Furthermore, it is to be understood that
the features of the various embodiments described herein are not
mutually exclusive and may exist in various combinations and
permutations. As used herein, the terms "substantially" and
"approximately" mean.+-.10%, and in some embodiments, .+-.5%. The
term "consists essentially of" means excluding other materials that
contribute to function, unless otherwise defined herein.
Nonetheless, such other materials may be present, collectively or
individually, in trace amounts. Herein, the terms "radiation" and
"light" are utilized interchangeably unless otherwise
indicated.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, like reference characters generally refer to the
same parts throughout the different views. Also, the drawings are
not necessarily to scale, emphasis instead generally being placed
upon illustrating the principles of the invention. In the following
description, various embodiments of the present invention are
described with reference to the following drawings, in which:
FIG. 1A is a schematic of a wavelength beam combining (WBC) method
along the array dimension of a single row of emitters in accordance
with embodiments of the invention;
FIG. 1B is a schematic of a WBC method along the array dimension of
a two-dimensional array of emitters in accordance with embodiments
of the invention;
FIG. 1C is a schematic of a WBC method along the stack dimension of
a two-dimensional array of emitters in accordance with embodiments
of the invention;
FIG. 2 is a schematic showing the effects of smile in a WBC method
along the stack dimension of a two-dimensional array of diode laser
emitters in accordance with embodiments of the invention;
FIG. 3A is a schematic of a WBC system including an optical rotator
selectively rotating a one-dimensional array of beams in accordance
with embodiments of the invention;
FIG. 3B is a schematic of a WBC system including an optical rotator
selectively rotating a two-dimensional array of beams in accordance
with embodiments of the invention;
FIG. 3C is a schematic of a WBC system including an optical rotator
selectively reorienting a two-dimensional array of beams in
accordance with embodiments of the invention;
FIG. 3D illustrates output profile views of the system of FIG. 3C
with and without an optical rotator in accordance with embodiments
of the invention;
FIGS. 4A-4C illustrate examples of optical rotators in accordance
with embodiments of the invention;
FIGS. 5A-5C illustrate related methods for placing combining
elements to generate one-dimensional or two-dimensional laser
elements;
FIG. 6 illustrates a WBC embodiment having a spatial repositioning
element in accordance with embodiments of the invention;
FIG. 7 illustrates an embodiment of a two-dimensional array of
emitters being reconfigured before a WBC step and individual beam
rotation after the WBC step in accordance with embodiments of the
invention;
FIG. 8 illustrates the difference between slow and fast WBC in
accordance with embodiments of the invention;
FIG. 9A illustrates embodiments using an optical rotator before WBC
in both a single and stacked array configurations in accordance
with embodiments of the invention;
FIG. 9B illustrates additional embodiments using an optical rotator
before WBC in accordance with embodiments of the invention;
FIG. 10 is illustrative of a single semiconductor chip emitter in
accordance with embodiments of the invention;
FIGS. 11A-11E illustrate an embodiment having spatial repositioning
elements upstream of the dispersive element that reduce the overall
beam quality factor of the combined beams in accordance with
embodiments of the invention; and
FIGS. 12A and 12B illustrate another embodiment reducing the beam
quality factor of the combined beams using angled gratings (FIG.
12A) to create distinct wavelength bands that are later combined
using dichroic mirrors (FIG. 12B).
DETAILED DESCRIPTION
Aspects and embodiments relate generally to the field of scaling
laser sources to high-power and high-brightness using an external
cavity and, more particularly, to methods and apparatus for
external-cavity beam combining using both one-dimensional or
two-dimensional laser sources. In one embodiment the external
cavity system includes one-dimensional or two-dimensional laser
elements, an optical system, a dispersive element, and a partially
reflecting element. An optical system is one or more optical
elements that perform two basic functions. The first function is to
overlap all the laser elements along the beam combining dimension
onto a dispersive element. The second function is to ensure all the
elements along the non-beam combining dimension are propagating
normal to the output coupler. In various embodiments, the optical
system introduces as little loss as possible. As such, these two
functions will enable a single resonance cavity for all the laser
elements.
In another embodiment the WBC external cavity system includes
wavelength stabilized one-dimensional or two-dimensional laser
elements, an optical system, and a dispersive element.
One-dimensional or two-dimensional wavelength stabilized laser
elements, with unique wavelength, can be accomplished using various
means such as laser elements with feedback from wavelength chirped
Volume Bragg grating, distributed feedback (DFB) laser elements, or
distributed Bragg reflector (DBR) laser elements. Here the main
function of the optical system is to overlap all the beams onto a
dispersive element. When there is no output coupler mirror external
to the wavelength-stabilized laser element, having parallel beams
along the non-beam-combining dimension is less important. Aspects
and embodiments further relate to high-power and/or high-brightness
multi-wavelength external-cavity lasers that generate an
overlapping or coaxial beam from very low output power to hundreds
and even to megawatts of output power.
In particular, aspects and embodiments are directed to a method and
apparatus for manipulating the beams emitted by the laser elements
of these external-cavity systems and combining them using a WBC
method to produce a desired output profile. Wavelength beam
combining methods have been developed to combine asymmetrical beam
elements across their respective slow or fast axis dimension. One
advantage of embodiments of the present invention is the ability to
selectively-reconfigure input beams either spatially or by
orientation to be used in slow and fast axis WBC methods, as well
as a hybrid of the two. Another advantage is the ability to
selectively-reconfigure input beams when there is a fixed-position
relationship to other input beams.
FIG. 1A illustrates a basic WBC architecture. In this particular
illustration, WBC is performed along the array dimension or slow
dimension for broad-area emitters. Individual beams 104 are
illustrated in the figures by a dash or single line, where the
length or longer dimension of the beam represents the array
dimension or slow diverging dimension for broad-area emitters and
the height or shorter dimension represents the fast diverging
dimension. (See also the left side of FIG. 8). In this related art,
a diode bar 102 having four emitters is illustrated. The emitters
are aligned in a manner such that the slow dimension ends of each
emitted beam 104 are aligned to one another side by side along a
single row--sometimes referred to as an array. However, it is
contemplated that any lasing elements may be used and in particular
laser elements with broad gain bandwidth. Typically a collimation
lens 106 is used to collimate each beam along the fast diverging
dimension. In some cases the collimation optics can be composed of
separate fast axis collimation lenses and slow axis collimation
lenses. Typically, transform optic 108 is used to combine each beam
along the WBC dimension 110 as shown by the input front view 112.
Transform optic 108 may be a cylindrical or spherical lens or
mirror. The transform optic 108 then overlaps the combined beam
onto a dispersive element 114 (here shown as a reflecting
diffraction grating). The first-order diffracted beams are incident
onto a partially reflecting mirror. The laser resonator is formed
between the back facet of the laser elements and the partially
reflecting mirror. As such, the combined beam is then transmitted
as a single output profile onto an output coupler 116. This output
coupler then transmits the combined beams 120, as shown by the
output front view 118. It is contemplated creating a system devoid
of an output coupler. For instance, a one-dimensional or
two-dimensional system with wavelength stabilized laser elements
and each having a unique wavelength may be accomplished in a few
ways. One system or method uses laser elements with feedback from
an external wavelength chirped Volume Bragg grating along the beam
combining dimension. Another uses internal distributed feedback
(DFB) laser elements or internal distributed Bragg reflector (DBR)
laser elements. In these systems, the single output profile
transmitted from the dispersive element would have the same profile
as 118. The output coupler 116 may be a partially reflective mirror
or surface or optical coating and act as a common front facet for
all the laser elements in diode array 102. A portion of the emitted
beams is reflected back into the optical gain and/or lasing portion
of diode array 102 in this external cavity system 100a. An external
cavity is a lasing system where the secondary mirror is displaced
at a distance away from the emission aperture or facet (not
labeled) of each laser emitter. Generally, in an external cavity
additional optical elements are placed between the emission
aperture or facet and the output coupler or partially reflective
surface.
Similarly, FIG. 1B illustrates a stack of laser diode bars each
having four emitters where those bars are stacked three high. Like
FIG. 1A, the input front view 112 of FIG. 1B, which in this
embodiment is a two-dimensional array of emitters, is combined to
produce the output front view 118 or a single column of emitters
120. The emitted beams in external cavity 100b were combined along
the array dimension. Here transform optic 108 is a cylindrical lens
used to combine the beams along the array. However, a combination
of optical elements or optical system may be used as such that the
optical elements arrange for all the beams to overlap onto the
dispersive element and ensure all the beams along the
non-beam-combining dimension are propagating normal to the output
coupler. A simple example of such an optical system would be a
single cylindrical lens with the appropriate focal length along the
beam-combining dimension and two cylindrical lenses that form an
afocal telescope along the non-beam-combining dimension wherein the
optical system projects images onto the partially reflecting
mirrors. Many variations of this optical system can be designed to
accomplish the same functions.
The array dimension FIG. 1B is also the same axis as the slow
dimension of each emitted beam in the case of multimode diode laser
emitters. Thus, this WBC system may also be called slow axis
combining, where the combining dimension is the same dimension of
the beams.
By contrast, FIG. 1C illustrates a stack 150 of laser diode arrays
102 forming a two-dimensional array of emitters, as shown by 120,
where instead of combining along the array dimension as in FIGS. 1A
and 1B, the WBC dimension now follows along the stack dimension of
the emitters. Here, the stacking dimension is also aligned with the
fast axis dimension of each of the emitted beams. The input front
view 112 is now combined to produce the output front view 118
wherein a single column 120 of emitters is shown.
There are various drawbacks to all three configurations. One of the
main drawbacks of configuration shown in FIGS. 1A and 1B is that
beam combining is performed along the array dimension. As such
external-cavity operation is highly dependent on imperfections of
the diode array. If broad-area semiconductor laser emitters are
used the spectral utilization in the WBC system is not as efficient
as if beam combining is performed along the fast axis dimension.
One of the main drawbacks of configurations shown in FIG. 1C is
that external beam shaping for beam symmetrization is required for
efficient coupling into a fiber. The beam symmetrization optics
needed for a high power system having a large number of emitters
may be complex and non-trivial. Another disadvantage of
configuration 1C is that the output beam quality is limited to that
of a single laser bar. Typical semiconductor or diode laser bars
have 19 to 49 emitters per bar with nearly diffraction-limited beam
quality in one dimension and beam quality that is several hundreds
of times diffraction-limited along the array dimension. After beam
symmetrization the output beam 120 can be coupled into at best a
100 .mu.m/0.22 Numerical Aperture (NA) fiber. To obtain higher beam
quality a small number of emitter bars is needed. For example to
couple into 50 .mu.m/0.22 NA fiber a five-emitter output beam is
needed. In many industrial laser applications a higher brightness
laser beam is required. For example, in some applications a
two-emitter output beam is needed instead of 19 or 49. The
two-emitter output beam can be coupled to a smaller core diameter
fiber with much more engineering tolerance and margin. This
additional margin in core diameter and NA is critical for reliable
operation at high power (kW-class) power levels. While it is
possible to procure five-emitter or two-emitter bars the cost and
complexity is generally much higher as compare to a standard 19 or
49 emitter bars because of the significantly reduced power per bar.
In this disclosure, we disclose methods to remove all of the above
shortcomings.
The previous illustrations, FIGS. 1A-1C, showed pre-arranged or
fixed position arrays and stacks of laser emitters. Generally,
arrays or stacks are arranged mechanically or optically to produce
a particular one-dimensional or two-dimensional profile. Thus,
fixed-position is used to describe a preset condition of laser
elements where the laser elements are mechanically fixed with
respect to each other as in the case of semiconductor or diode
laser bars having multiple emitters or fiber lasers mechanically
spaced apart in V-grooves, as well as other laser emitters that
come packaged with the emitters in a fixed position.
Alternatively, fixed position may refer to the secured placement of
a laser emitter in a WBC system where the laser emitter is
immobile. Pre-arranged refers to an optical array or profile that
is used as the input profile of a WBC system. Often times the
pre-arranged position is a result of emitters configured in a
mechanically fixed position. Pre-arranged and fixed position may
also be used interchangeably. Examples of fixed-position or
pre-arranged optical systems are shown in FIGS. 5A-C.
FIGS. 5A-5C refer to prior art illustrated examples of optically
arranged one and two-dimensional arrays. FIG. 5A illustrates an
optically arranged stack of individual optical elements 510.
Mirrors 520 are used to arrange the optical beams from optical
elements 530, each optical element 530 having a near field image
540, to produce an image 550 (which includes optical beams from
each optical element 530) corresponding to a stack 560 (in the
horizontal dimension) of the individual optical elements 510.
Although the optical elements 500 may not be arranged in a stack,
the mirrors 520 arrange the optical beams such that the image 550
appears to correspond to the stack 560 of optical elements 510.
Similarly, in FIG. 5B, the mirrors 520 can be used to arrange
optical beams from diode bars or arrays 570 to create an image 550
corresponding to a stack 560 of diode bars or arrays 575. In this
example, each diode bar or array 570 has a near field image 540
that includes optical beams 545 from each individual element in the
bar or array. Similarly, the minors 520 may also be used to
optically arrange laser stacks 580 into an apparent larger overall
stack 560 of individual stacks 585 corresponding to image 550, as
shown in FIG. 5C.
Nomenclature, used in prior art to define the term "array
dimension," referred to one or more laser elements placed side by
side where the array dimension is also along the slow axis. One
reason for this nomenclature is diode bars with multiple emitters
are often arranged in this manner where each emitter is aligned
side by side such that each beam's slow dimension is along a row or
array. For purposes of this application, an array or row refers to
individual emitters or beams arranged across a single dimension.
The individual slow or fast dimension of the emitters of the array
may also be aligned along the array dimension, but this alignment
is not to be assumed. This is important because some embodiments
described herein individually rotate the slow dimension of each
beam aligned along an array or row. Additionally, the slow axis of
a beam refers to the wider dimension of the beam and is typically
also the slowest diverging dimension, while the fast axis refers to
the narrower dimension of the beam and is typically the fastest
diverging dimension. The slow axis may also refer to single mode
beams
Additionally, some prior art defines the term "stack or stacking
dimension" referred to as two or more arrays stacked together,
where the beams' fast dimension is the same as the stacking
dimension. These stacks were pre-arranged mechanically or
optically. However, for purposes of this application a stack refers
to a column of beams or laser elements and may or may not be along
the fast dimension. Particularly, as discussed above, individual
beams or elements may be rotated within a stack or column.
In some embodiments it is useful to note that the array dimension
and the slow dimension of each emitted beam are initially oriented
across the same axis; however, those dimensions, as described in
this application, may become oriented at an offset angle with
respect to each other. In other embodiments, the array dimension
and only a portion of the emitters arranged along the array or
perfectly aligned the same axis at a certain position in a WBC
system. For example, the array dimension of a diode bar may have
emitters arranged along the array dimension, but because of smile
(often a deformation or bowing of the bar) individual emitters'
slow emitting dimension is slightly skewed or offset from the array
dimension.
Laser sources based on common "commercial, off-the-shelf" or COTS
high power laser diode arrays and stacks are based on broad-area
semiconductor or diode laser elements. Typically, the beam quality
of these elements is diffraction-limited along the fast axis and
many times diffraction-limited along the slow axis of the laser
elements. It is to be appreciated that although the following
discussion may refer primarily to single emitter laser diodes,
diode laser bars and diode laser stacks, embodiments of the
invention are not limited to semiconductor or laser diodes and may
be used with many different types of laser and amplifier emitters,
including fiber lasers and amplifiers, individually packaged diode
lasers, other types of semiconductor lasers including quantum
cascade lasers (QCLs), tapered lasers, ridge waveguide (RWG)
lasers, distributed feedback (DFB) lasers, distributed Bragg
reflector (DBR) lasers, grating coupled surface emitting laser,
vertical cavity surface emitting laser (VCSEL), and other types of
lasers and amplifiers.
All of the embodiments described herein can be applied to WBC of
diode laser single emitters, bars, and stacks, and arrays of such
emitters. In those embodiments employing stacking of diode laser
elements, mechanical stacking or optical stacking approaches can be
employed. In addition, where an HR coating is indicated at the
facet of a diode laser element, the HR coating can be replaced by
an AR coating, provided that external cavity optical components,
including but not limited to a collimating optic and bulk HR mirror
are used in combination with the AR coating. This approach is used,
for example, with WBC of diode amplifier elements. Slow axis is
also defined as the worse beam quality direction of the laser
emission. The slow axis typically corresponds to the direction
parallel to the semiconductor chip at the plane of the emission
aperture of the diode laser element. Fast axis is defined as the
better beam quality direction of the laser emission. Fast axis
typically corresponds to the direction perpendicular to the
semiconductor chip at the plane of the emission aperture of the
diode laser element.
An example of a single semiconductor chip emitter 1000 is shown in
FIG. 10. The aperture 1050 is also indicative of the initial beam
profile. Here, the height 1010 at 1050 is measured along the stack
dimension. Width 1020 at 1050 is measured along the array
dimension. Height 1010 is the shorter dimension at 1050 than width
1020. However, height 1010 expands faster or diverges to beam
profile 1052, which is placed at a distance away from the initial
aperture 1050. Thus, the fast axis is along the stack dimension.
Width 1020 which expands or diverges at a slower rate as indicated
by width 1040 being a smaller dimension than height 1030. Thus, the
slow axis of the beam profile is along the array dimension. Though
not shown, multiple single emitters such as 1000 may be arranged in
a bar side by side along the array dimension.
Drawbacks for combining beams primarily along their slow axis
dimension may include: reduced power and brightness due to lasing
inefficiencies caused by pointing errors, smile and other
misalignments. As illustrated in FIG. 2, a laser diode array with
smile, one often caused by the diode array being bowed in the
middle sometimes caused by the diode laser bar mounting process, is
one where the individual emitters along the array form a typical
curvature representative of that of a smile. Pointing errors are
individual emitters along the diode bar emitting beams at an angle
other than normal from the emission point. Pointing error may be
related to smile, for example, the effect of variable pointing
along the bar direction of a diode laser bar with smile when the
bar is lensed by a horizontal fast axis collimating lens. These
errors cause feedback from the external cavity, which consists of
the transform lens, grating, and output coupler, not to couple back
to the diode laser elements. Some negative effects of this
miscoupling are that the WBC laser breaks wavelength lock and the
diode laser or related packaging may be damaged from miscoupled or
misaligned feedback not re-entering the optical gain medium. For
instance the feedback may hit some epoxy or solder in contact or in
close proximity to a diode bar and cause the diode bar to fail
catastrophically.
Row 1 of FIG. 2 shows a single laser diode bar 202 without any
errors. The embodiments illustrated are exemplary of a diode bar
mounted on a heat sink and collimated by a fast-axis collimation
optic 206. Column A shows a perspective or 3-D view of the
trajectory of the output beams 211 going through the collimation
optic 206. Column D shows a side view of the trajectory of the
emitted beams 211 passing through the collimation optic 206. Column
B shows the front view of the laser facet with each individual
laser element 213 with respect to the collimation optic 206. As
illustrated in row 1, the laser elements 213 are perfectly
straight. Additionally, the collimation optic 206 is centered with
respect to all the laser elements 213. Column C shows the expected
output beam from a system with this kind of input. Row 2
illustrates a diode laser array with pointing error. Shown by
column B of row 2 the laser elements and collimation optic are
slightly offset from each other. The result, as illustrated, is the
emitted beams having an undesired trajectory that may result in
reduced lasing efficiency for an external cavity. Additionally, the
output profile may be offset to render the system ineffective or
cause additional modifications. Row 3 shows an array with packaging
error. The laser elements no longer sit on a straight line, and
there is curvature of the bar. This is sometimes referred to as
"smile." As shown on row 3, smile may introduce even more
trajectory problems as there is no uniform path or direction common
to the system. Column D of row 3 further illustrates beams 211
exiting at various angles. Row 4 illustrates a collimation lens
unaligned with the laser elements in a twisted or angled manner.
The result is probably the worst of all as the output beams
generally have the most collimation or twisting errors. In most
systems, the actual error in diode arrays and stacks is a
combination of the errors in rows 2, 3, and 4. In both methods 2
and 3, using VBG's and diffraction gratings, laser elements with
imperfections result in output beams no longer pointing parallel to
the optical axis. These off optical axis beams result in each of
the laser elements lasing at different wavelengths. The plurality
of different wavelengths increases the output spectrum of the
system to become broad as mentioned above.
One of the advantages of performing WBC along the stacking
dimension (here also primarily the fast dimension) of a stack of
diode laser bars is that it compensates for smile as shown in FIG.
2. Pointing and other alignment errors are not compensated by
performing WBC along the array dimension (also primarily slow
dimension). A diode bar array may have a range of emitters
typically from 19 to 49 or more. As noted, diode bar arrays are
typically formed such that the array dimension is where each
emitter's slow dimension is aligned side by side with the other
emitters. As a result, when using WBC along the array dimension,
whether a diode bar array has 19 or 49 emitters (or any other
number of emitters), the result is that of a single emitter. By
contrast, when performing WBC along the orthogonal or fast
dimension of the same single diode bar array, the result is each
emitted beam increases in spectral brightness, or narrowed spectral
bandwidth, but there is not a reduction in the number of beams
(equivalently, there is not an increase in spatial brightness).
This point is illustrated in FIG. 8. On the left of FIG. 8 is shown
a front view of an array of emitters 1 and 2 where WBC along the
slow dimension is being performed. Along the right side using the
same arrays 1 and 2, WBC along the fast dimension is being
performed. When comparing array 1, WBC along the slow dimension
reduces the output profile to that of a single beam, while WBC
along the fast dimension narrows the spectral bandwidth, as shown
along the right side array 1, but does not reduce the output
profile size to that of a single beam.
Using COTS diode bars and stacks the output beam from beam
combining along the stack dimension is usually highly asymmetric.
Symmetrization, or reducing the beam profile ratio closer to
equaling one, of the beam profile is important when trying to
couple the resultant output beam profile into an optical fiber.
Many of the applications of combining a plurality of laser emitters
require fiber coupling at some point in an expanded system. Thus,
having greater control over the output profile is another advantage
of the application.
Further analyzing array 2 in FIG. 8 shows the limitation of the
number of emitters per laser diode array that is practical for
performing WBC along the fast dimension if very high brightness
symmetrization of the output profile is desired. As discussed
above, typically the emitters in a laser diode bar are aligned side
by side along their slow dimension. Each additional laser element
in a diode bar is going to increase the asymmetry in the output
beam profile. When performing WBC along the fast dimension, even if
a number of laser diode bars are stacked on each other, the
resultant output profile will still be that of a single laser diode
bar. For example if one uses a COTS 19-emitter diode laser bar, the
best that one can expect is to couple the output into a 100
.mu.m/0.22 NA fiber. Thus, to couple into a smaller core fiber
lower number of emitters per bar is required. One could simply fix
the number of emitters in the laser diode array to 5 emitters in
order to help with the symmetrization ratio; however, fewer
emitters per laser diode bar array generally results in an increase
of cost of per bar or cost per Watt of output power. For instance,
the cost of diode bar having 5 emitters may be around $2,000
whereas the cost of diode bar having 49 emitters may be around
roughly the same price. However, the 49 emitter bar may have a
total power output of up to an order-of-magnitude greater than that
of the 5 emitter bar. Thus, it would be advantageous for a WBC
system to be able to achieve a very high brightness output beams
using COTS diode bars and stacks with larger number of emitters. An
additional advantage of bars with larger number of emitters is the
ability to de-rate the power per emitter to achieve a certain power
level per bar for a given fiber-coupled power level, thereby
increasing the diode laser bar lifetime or bar reliability.
One embodiment that addresses this issue is illustrated in FIG. 3A,
which shows a schematic of WBC system 300a with an optical rotator
305 placed after collimation lenses 306 and before the transform
optic 308. It should be noted the transform optic 308 may include
or consist essentially of a number of lenses or mirrors or other
optical components. The optical rotator 305 individually rotates
the fast and slow dimension of each emitted beam shown in the input
front view 312 to produce the re-oriented front view 307. It should
be noted that the optical rotators can selectively rotate each beam
individually irrespective of the other beams or can rotate all the
beams through the same angle simultaneously. It should also be
noted that a cluster of two or more beams can be rotated
simultaneously. The resulting output after WBC is performed along
the array dimension is shown in output front view 318 as a single
emitter. Dispersive element 314 is shown as a reflection
diffraction grating, but may also be a dispersive prism, a grism
(prism+grating), transmission grating, and Echelle grating. This
particular embodiment illustrated shows only four laser emitters;
however, as discussed above this system could take advantage of a
laser diode array that included many more elements, e.g., 49. This
particular embodiment illustrated shows a single bar at a
particular wavelength band (example at 976 nm) but in actual
practice it may be composed of multiple bars, all at the same
particular wavelength band, arranged side-by-side. Furthermore,
multiple wavelength bands (example 976 nm, 915 nm, and 808 nm),
with each band composing of multiple bars, may be combined in a
single cavity. Because WBC was performed across the fast dimension
of each beam it easier to design a system with a higher brightness
(higher efficiency due to insensitivity due to bar imperfections);
additionally, narrower bandwidth and higher power output are all
achieved. As previously discussed it should be noted that some
embodiments WBC system 300a may not include output coupler 316
and/or collimation lens(es) 306. Furthermore, pointing errors and
smile errors are compensated for by combining along the stack
dimension (In this embodiment this is also the fast dimension).
FIG. 3B, shows an implementation similar to 3A except that a stack
350 of laser arrays 302 forms a 2-D input profile 312. Cavity 300b
similarly consists of collimation lens(es) 306, optical rotator
305, transform optic 308, dispersive element 308 (here a
diffraction grating), and an output coupler 316 with a partially
reflecting surface. Each of the beams is individually rotated by
optical rotator 305 to form an after rotator profile 307. The WBC
dimension is along the array dimension, but with the rotation each
of the beams will be combined across their fast axis. Fast axis WBC
produces outputs with very narrow line widths and high spectral
brightness. These are usually ideal for industrial applications
such as welding. After transform optic 308 overlaps the rotated
beams onto dispersive element 314 a single output profile is
produced and partially reflected back through the cavity into the
laser elements. The output profile 318 is now comprised of a line
of three (3) beams that is quite asymmetric.
FIG. 3C shows the same implementation when applied to 2-D laser
elements. The system consists of 2-D laser elements 302, optical
rotator 305, transform optical system (308 and 309a-b) a dispersive
element 314, and a partially reflecting mirror 316. FIG. 3C
illustrates a stack 350 of laser diode bars 302 with each bar
having an optical rotator 305. Each of the diode bars 302 (three
total) as shown in external cavity 300c includes four emitters.
After input front view 312 is reoriented by optical rotator 305,
reoriented front view 307 now the slow dimension of each beam
aligned along the stack dimension. WBC is performed along the
dimension, which is now the slow axis of each beam and the output
front view 318 now comprises single column of beams with each
beam's slow dimension oriented along the stack dimension. Optic
309a and 309b provide a cylindrical telescope to image along the
array dimension. The function of the three cylindrical lenses is
two-fold. The middle cylindrical lens is the transform lens and its
main function is to overlap all the beams onto the dispersive
element. The two other cylindrical lenses 309a and 309b form an
afocal cylindrical telescope along the non-beam combining
dimension. Its main function is to make sure all laser elements
along the non-beam combining are propagation normal to the
partially reflecting mirror. As such the implementation as shown in
FIG. 3C has the same advantages as the one shown in FIG. 1C.
However, unlike the implementation as shown in FIG. 1C the output
beam is not the same as the input beam. The number of emitters in
the output beam 318 in FIG. 3C is the same as the number of bars in
the stack. For example, if the 2-D laser source consists of a
three-bar stack with each bar composed of 49 emitters, then the
output beam in FIG. 1C is a single bar with 49 emitters. However,
in FIG. 3C the output beam is a single bar with only three
emitters. Thus, the output beam quality or brightness is more than
one order of magnitude higher. This brightness improvement is very
significant for fiber-coupling. For higher power and brightness
scaling multiple stacks can be arranged side-by-side.
To illustrate this configuration further, for example, assume WBC
is to be performed of a three-bar stack, with each bar comprising
of 19 emitters. So far, there are three options. First, wavelength
beam combining can be performed along the array dimension to
generate three beams as shown in FIG. 1B. Second, wavelength beam
combining can be performed along the stack dimension to generate 19
beams a shown FIG. 1C. Third, wavelength beam combining can be
performed along the array dimension using beam rotator to generate
19 beams as shown FIG. 3C. There are various trade-offs for all
three configuration. The first case gives the highest spatial
brightness but the lowest spectral brightness. The second case
gives the lowest spatial brightness with moderate spectral
brightness and beam symmetrization is not required to couple into a
fiber. The third case gives the lowest spatial brightness but the
highest spectral brightness and beam symmetrization is required to
couple into an optical fiber. In some applications this more
desirable.
To illustrate the reduction in asymmetry FIG. 3D has been drawn
showing the final output profile 319a where the system of 300b did
not have an optical rotator and output profile 319b where the
system includes an optical rotator. Though these figures are not
drawn to scale, they illustrate an advantage achieved by utilizing
an optical rotator, in a system with this configuration where WBC
is performed across the slow dimension of each beam. The shorter
and wider 319b is more suitable for fiber coupling than the taller
and slimmer 319a.
Examples of various optical rotators are shown in FIG. 4A-4C. FIG.
4A illustrates an array of cylindrical lenses (419a and 419b) that
cause input beam 411a to be rotated to a new orientation at 411b.
FIG. 4B similarly shows input 411a coming into the prism at an
angle, which results in a new orientation or rotation beam 411b.
FIG. 4C illustrates an embodiment using a set of step mirrors 417
to cause input 411a to rotate at an 80-90 degree angle with the
other input beams resulting in a new alignment of the beams 411b
where they are side by side along their respective fast axis. These
devices and others may cause rotation through both non-polarization
sensitive as well as polarization sensitive means. Many of these
devices become more effective if the incoming beams are collimated
in at least the fast dimension. It is also understand that the
optical rotators can selectively rotate the beams at various
including less than 90 degrees, 90 degrees and greater than 90
degrees.
The optical rotators in the previous embodiments may selectively
rotate individual, rows or columns, and groups of beams. In some
embodiments a set angle of rotation, such as a range of 80-90
degrees is applied to the entire profile or subset of the profile.
In other instances, varying angles of rotation are applied uniquely
to each beam, row, column or subset of the profile (see FIGS.
9A-B). For instance, one beam may be rotated by 45 degrees in a
clockwise direction while an adjacent beam is rotated 45 degrees in
a counterclockwise direction. It is also contemplated one beam is
rotated 10 degrees and another is rotated 70 degrees. The
flexibility the system provides may be applied to a variety of
input profiles, which in turn helps determine how the output
profile is to be formed.
Performing WBC along an intermediate angle between the slow and
fast dimension of the emitted beams is also well within the scope
of the invention (See for example 6 on FIG. 9B). Some laser
elements as described herein, produce electromagnetic radiation and
include an optical gain medium. When the radiation or beams exit
the optical gain portion they generally are collimated along the
slow and/or fast dimension through a series of micro lenses. From
this point, the embodiments already described in this section
included an optical rotator that selectively and rotated each beam
prior to the beams being overlapped by a transform lens along
either the slow or the fast dimension of each beam onto a
dispersive element. The output coupler may or may not be coated to
partially reflect the beams back into the system to the laser
element where the returned beams assist in generating more external
cavity feedback in the optical gain element portion until they are
reflected off a fully reflective mirror in the back portion of the
laser element. The location of the optical elements listed above
and others not listed are with respect to the second partially
reflective surface helps decide whether the optical elements are
within an external cavity system or outside of the lasing cavity.
In some embodiments, not shown, the second partially reflective
mirror resides at the end of the optical gain elements and prior to
the collimating or rotating optics.
Another method for manipulating beams and configurations to take
advantage of the various WBC methods includes using a spatial
repositioning element. This spatial repositioning element may be
placed in an external cavity at a similar location as to that of an
optical rotator. For example, FIG. 6 shows a spatial repositioning
element 603 placed in the external cavity WBC system 600 after the
collimating lenses 606 and before the transform optic(s) 608. The
purpose of a spatial repositioning element is to reconfigure an
array of elements into a new configuration. FIG. 6 shows a
three-bar stack with N elements reconfigured to a six-bar stack
with N/2 elements. Spatial repositioning is particularly useful in
embodiments such as 600, where stack 650 is a mechanical stack or
one where diode bar arrays 602 and their output beams were placed
on top of each other either mechanically or optically. With this
kind of configuration the laser elements have a fixed-position to
one another. Using a spatial repositioning element can form a new
configuration that is more ideal for WBC along the fast dimension.
The new configuration makes the output profile more suitable for
fiber coupling.
For example, FIG. 7 illustrates an embodiment wherein a
two-dimensional array of emitters 712 is reconfigured during a
spatial repositioning step 703 by a spatial repositioning optical
element such as an array of periscope mirrors. The reconfigured
array shown by reconfigured front view 707 is now ready for a WBC
step 710 to be performed across the WBC dimension, which here is
the fast dimension of each element. The original two-dimensional
profile in this example embodiment 700 is an array of 12 emitters
tall and 5 emitters wide. After the array is transmitted or
reflected by the spatial repositioning element a new array of 4
elements tall and 15 elements wide is produced. In both arrays the
emitters are arranged such that the slow dimension of each is
vertical while the fast dimension is horizontal. WBC is performed
along the fast dimension which collapses the 15 columns of emitters
in the second array into 1 column that is 4 emitters tall. This
output is already more symmetrical than if WBC had been performed
on the original array, which would have resulted in a single column
15 emitters tall. As shown, this new output may be further
symmetrized by an individually rotating step 705 rotating each
emitter by 90 degrees. In turn, a post-WBC front view 721 is
produced being the width of a single beam along the slow dimension
and stacked 4 elements high, which is a more suitable for coupling
into a fiber.
One way of reconfiguring the elements in a one-dimensional or
two-dimensional profile is to make `cuts` or break the profile into
sections and realign each section accordingly. For example, in FIG.
7 two cuts 715 were made in 713. Each section was placed side by
side to form 707. These optical cuts can be appreciated if we note
the elements of 713 had a pre-arranged or fixed-position
relationship. It is also well within the scope to imagine any
number of cuts being made to reposition the initial input beam
profile. Each of these sections may in addition to being placed
side by side, but on top and even randomized if so desired.
Spatial repositioning elements may be comprised of a variety of
optical elements including periscope optics that are both polarized
and non-polarized as well as other repositioning optics. Step
mirrors as shown in FIG. 4a may also be reconfigured to become a
spatial repositioning element.
In another embodiment illustrated in FIGS. 11A-11E, a laser
apparatus 1100 features a collection of multiple beam emitters 102
each emitting a laser beam and that collectively form an array
having first and second orthogonal dimensions. The laser apparatus
110 also includes focusing optics 108 (e.g., one or more
cylindrical lenses and/or mirrors, and/or one or more spherical
lenses and/or mirrors) that combine the beams emitted by the
emitters 102 along the first dimension to form a combined beam 1110
that propagates toward a dispersive element 114. The dispersive
element 114 (e.g., a diffraction grating, a dispersive prism, a
grism (prism/grating), a transmission grating, or an Echelle
grating) may be disposed approximately at the focal length of the
focusing optics 108. As described herein, the dispersive element
114 receives the combined beam and transmits the beam as a
multi-wavelength beam having a high brightness. The
multi-wavelength beam is transmitted to a partially reflective
output coupler (not shown in FIGS. 11A-11E) that transmits a
portion of the multi-wavelength beam (e.g., to a workpiece) and
reflects a second portion of the multi-wavelength beam back toward
the dispersive element 114 and thence to the emitters 102, forming
an external lasing cavity.
In various embodiments of the present invention, laser apparatus
1100 also includes multiple optical repositioners, each of which
may include or consist essentially of one or more reflectors 1120
and one or more optical alignment elements 1130 (as shown in more
detail in FIG. 11D). Each reflector 1120 may include or consist
essentially of a mirror or an object with a reflective surface, and
each optical alignment element 1130 may include or consist
essentially of, e.g., a lens, prism, or a substantially transparent
solid body that bends incoming light, transmits it, and redirects
the transmitted light along a path or direction different from that
of the incident light. The optical repositioners each intercept a
portion of the combined beam along the second dimension downstream
of the focusing optics 108 but upstream of the dispersive element
114. As shown, a portion of the combined beam propagates to the
dispersive element 114 without being defected or intercepted by an
optical repositioner. The dimensions of this unintercepted portion
of the combined beam may correspond to those of the desired final
multi-wavelength beam. Thus, the optical respositioners redirect
the portions of the combined beam to at least partially overlap
with the unintercepted portion of the combined beam at the
dispersive element 114; this may thereby reduce the size of the
beam at the dispersive element 114 along the second dimension to
approximately the dimension of the unintercepted portion. For
example, as shown in FIGS. 11A-11D, in an embodiment the optical
repositioners each intercept approximately one-third of the
combined beam along the second dimension, reducing the beam size
along the second dimension to about one-third of that of the
initial combined beam. Since the focusing optics 108 reduce the
size of the combined beam along the first dimension, e.g., to the
size of a single emitter, the size (and thus beam quality factor)
of the final beam may be reduced considerably. While FIGS. 11A-11D
show each optical repositioner intercepting approximately one-third
of the combined beam, embodiments of the invention features optical
respositioners intercepting any fraction of the combined beam.
FIGS. 12A and 12B illustrate a laser apparatus 1200, in accordance
with embodiments of the present invention, that utilizes angled
gratings aligned along the beam combining line. As shown in FIG.
12A, a group of beam emitters 102 (e.g., a two-dimensional array of
emitters) emits beams that encounter focusing optics 108 that
combine the beams along one dimension onto a series of gratings
1210-1, 1210-2, 1210-3 disposed at different angles with respect to
the beam. FIG. 12A shows three gratings that pivot about the
beam-combining point and result in a series of combined
multi-wavelength beams 1220 being produced. As shown, each grating
intercepts only a portion of the combined beam from the focusing
optics 108 at a different angle of incidence. This incidence angle
onto the grating causes each portion of the combined beam to
produce a unique band of wavelengths transmitted by the partially
reflective output coupler 116 as transmitted beam 1230.
Effectively, each band is a multi-wavelength beam including light
of different wavelengths.
As shown in FIG. 12B, which illustrates a laser apparatus 1250
(which may be a portion of a larger laser apparatus along with
laser apparatus 1200) the fact that transmitted beam 1230 includes
multiple distinct bands of wavelengths enables separation and
recombination of transmitted beam 1230 via the use of dichroic
elements 1260 that selectively reflect and/or transmit one or more
of the distinct bands. As shown, in an exemplary embodiment, the
transmitted beam 1230 has three distinct bands (resulting from,
e.g., three different angled gratings as shown in FIG. 12A), and
the dichroic elements 1260 may be utilized to separate the
different bands (labeled as #1, #2, and #3 in FIG. 12B) from each
other and then overlap the three bands into a single overlapping
beam 1270 with a reduced beam quality factor. The dichroic elements
1260 may include or consist essentially of, for example, dichroic
mirrors, dichroic filters, interference filters, etc. that are
configured to pass selected wavelengths of light while reflecting
others.
Additional embodiments of the invention are illustrated in FIGS.
9A-9B. The system shown in 1 of FIG. 9A shows a single array of
four beams aligned side to side along the slow dimension. An
optical rotator individually rotates each beam. The beams are then
combined along the fast dimension and are reduced to a single beam
by WBC. In this arrangement it is important to note that the 4
beams could easily be 49 or more beams. It may also be noted that
if some of the emitters are physically detached from the other
emitters, the individual emitter may be mechanically rotated to be
configured in an ideal profile. A mechanical rotator may be
comprised of a variety of elements including friction sliders,
locking bearings, tubes, and other mechanisms configured to rotate
the laser element. Once a desired position is achieved the laser
elements may then be fixed into place. It is also conceived that an
automated rotating system that can adjust the beam profile
depending on the desired profile may be implemented. This automated
system may either mechanically reposition a laser or optical
element or a new optical element may be inserted in and out of the
system to change the output profile as desired.
System 2 shown in FIG. 9A, shows a two-dimensional array having
three stacked arrays with four beams each aligned along the slow
dimension. (Similar to FIG. 3C) As this stacked array passes
through an optical rotator and WBC along the fast dimension a
single column of three beams tall aligned top to bottom along the
slow dimension is created. Again it is appreciated that if the
three stacked arrays shown in this system had 50 elements, the same
output profile would be created, albeit one that is brighter and
has a higher output power.
System 3 in FIG. 9B, shows a diamond pattern of four beams wherein
the beams are all substantially parallel to one another. This
pattern may also be indicative of a random pattern. The beams are
rotated and combined along the fast dimension, which results in a
column of three beams aligned along the slow dimension from top to
bottom. Missing elements of diode laser bars and stacks due to
emitter failure or other reasons, is an example of System 3. System
4, illustrates a system where the beams are not aligned, but that
one beam is rotated to be aligned with a second beam such that both
beams are combined along the fast dimension forming a single beam.
System 4, demonstrates a number of possibilities that expands WBC
methods beyond using laser diode arrays. For instance, the input
beams in System 4 may be from carbon dioxide (CO.sub.2) lasers,
semiconductor or diode lasers, diode pumped fiber lasers,
lamp-pumped or diode-pumped Nd:YAG lasers, Disk Lasers, and so
forth. The ability to mix and match the type of lasers and
wavelengths of lasers to be combined is another advantage of
embodiments of the present invention.
System 5, illustrates a system where the beams are not rotated to
be fully aligned with WBC dimension. The result is a hybrid output
that maintains many of the advantages of WBC along the fast
dimension. In several embodiments the beams are rotated a full 90
degrees to become aligned with WBC dimension, which has often been
the same direction or dimension as the fast dimension. However,
System 5 and again System 6 show that optical rotation of the beams
as a whole (System 6) or individually (System 5) may be such that
the fast dimension of one or more beams is at an angle theta or
offset by a number of degrees with respect to the WBC dimension. A
full 90 degree offset would align the WBC dimension with the slow
dimension while a 45 degree offset would orient the WBC dimension
at an angle halfway between the slow and fast dimension of a beam
as these dimension are orthogonal to each other. In one embodiment,
the WBC dimension has an angle theta at approximately 3 degrees off
the fast dimension of a beam.
The terms and expressions employed herein are used as terms of
description and not of limitation, and there is no intention, in
the use of such terms and expressions, of excluding any equivalents
of the features shown and described or portions thereof, but it is
recognized that various modifications are possible within the scope
of the invention claimed.
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